The present invention provides an efficient method for solvent extraction of crude oil from oil-impregnated diatomite ore. The ore is comprised of a base of diatoms, hollow siliceous skeletons of single celled animals, which are cemented by oil and water into soft aggregates. One such diatomaceous ore deposit, which is impregnated with crude oil characteristic of the area, is located in the United States at McKittrick, Calif. Such deposits, in addition to the oil-impregnated diatomaceous particles, also contain some fine clay, slit and water. A typical such diatomite ore, which contains about 12 percent oil and 34 percent water strongly absorbed in the hollow diatom skeletons, is a friable solid, slightly unctuous but not damp.
I am aware of no prior art which relates specifically to the solvent extraction of oil from diatomite ore, although solvent extraction processes for separating heavy bitumens from tar sands have previously been proposed but have not gained commercial acceptance. Tar sands are defined as sands cemented by a bitumen too viscous to be recovered by conventional crude oil production methods. Although diatomite is sometimes classified as a tar sand, their properties from the point of view of solvent extraction are so different that such a classification is misleading and incorrect.
It is well known to the art that any solvent extraction process employing a volatile solvent which must be recovered comprises the steps of preparation for extraction, extraction, recovery of solvent from spent solids and recovery of solvent from the extract. Diatomite ore requires little preparation, since it is a soft friable solid which disintegrates when mixed with solvent. The present process employs countercurrent decantation to extract oil from diatomite ore with a hydrocarbon solvent. Countercurrent decantation, well known to those skilled in the art of extraction, is carried out in multiple stages, each of which comprises a mixer and a thickener. It is well known to recover solvent from spent solids by vaporization. However, diatomite is unique in that the individual particles are of microscopic size, so that desolventizing of dry diatomite is not practical because of the large amount of dust which would be entrained with the vapor. Recovery of solvent from a non-volatile liquid extract by multiple effect evaporation followed by steam stripping is known. Separation of solvent from crude oil poses the additional problem that there are low boilers in the crude oil, an appreciable amount of which distill with the solvent and must be separated from it before the solvent is recycled to the extraction system.